The inbred mouse strain RIIIS/J is a model for type 1 von Willebrand Disease (VWD), the most common bleeding disorder in humans. We previously found that the cause of low VWF in RIIIS/J is a regulatory mutation, Mvwfl, in the gene encoding an N-acetylgalactosaminyltransferase, GALGT2. Mvwfl causes a tissue-specific switch from the intestinal epithelial expression pattern observed in most mouse strains to a vascular endothelial pattern, resulting in aberrant post-translational modification of VWF and accelerated clearance. The precise cis-regulatory elements responsible for this remarkable switch have not yet been identified. We hypothesize that characterization of the regulatory element(s) responsible for the RIIIS/J switch will significantly advance our understanding of vascular and intestine specific gene expression programs. This project will utilize direct sequence analysis and engineered bacterial artificial chromosome systems (BACs) to identify the genomic regions responsible for the switch. Comparative sequence analysis of different Mvwfl mouse strains should determine the minimum length of the shared Mvwfl haplotype block. Chimeric RIIIS/J:C57BL6/J BACs will be engineered by swapping fragments of the RIIIS/J candidate sequence into a C57BL/6J BAC that is known to exhibit intestinal epithelial Galgt2 expression. Transgenic mice will be generated and tested for tissue patterns of Galgt2 expression. Constructs that confer the RIIIS/J Galgt2 expression pattern should define the region containing the critical regulatory element(s), which will be further characterized by additional BAC mutations. Our findings should provide new insight into the cis-regulatory elements required for endothelial gene expression, with potential broad implications for other tissue specific gene expression programs and vascular gene therapy.